1. Field of the Invention
This invention relates to the synthesis of ceramic oxides using the sol-gel process and, more particularly, to the synthesis of ceramic oxides having transition metals selectively incorporated within a ceramic oxide matrix. Still more particularly, this invention relates to the control of ceramic oxide particle size and crystallinity during synthesis to achieve specific physical properties in order to produce tailored oxides with specific particle morphology and near zero expansion optical ceramics.
2. Description of the Prior Art
The sol-gel process is a well-known technique for preparing metal oxides by the hydrolysis of a metal-organic compound to form a sol. The very small particles of colloidal metal oxides in the sol gather together into clusters or masses until they become large enough and sufficiently immobile to produce first a viscous liquid and then a solid colloidal gel structure. This technique has been applied not only to the preparation of single component oxide glasses, but also to the preparation of multicomponent oxide glasses.
In the Sol-Gel technique, it is also known that growth of metal oxide chains and networks can be achieved which eventually lead to gelation. Properties of gels produced in this manner are fundamentally different from gels produced in colloidal systems. Control of chemical polymerization, such as the acid and base catalyzed hydrolysis of tetraethylorthosilicate in alcoholic solutions, allows control of the form of the resulting polymer. It is known that the form of the resulting polymers in the tetraethylorthosilicate system is governed by the relative rates of reactions of hydrolysis and condensation. Undesirable colloidal solutions are formed in the tetraethylorthosilicate system when hydrolysis is rapid compared to condensation.
The prior art teaches the production of simple binary glasses such as Si and Ti or Zr oxides from alkoxide solutions. A gelling agent added to the solution increases the viscosity so that shaping or application of the solution on other surfaces can occur independently of the progress of hydrolysis of the alkoxide in the solution. The use of acid or base catalysts to achieve polymer growth and resulting gelation without formation of colloidal silica is not discussed. Other references also disclose methods for producing ceramic articles and glass compositions using the Sol-Gel techniques; however, none disclose using Sol-Gel techniques to control hydrolysis and polymerization to synthesize materials with a specific particle size distribution and crystal structure in order to achieve specific physical properties such as a low thermal expansion coefficient.
One reference does teach the use of an "effective catalytic amount of an acid hydrolysis catalyst." This amount is disclosed as the range of 1 to about 500 parts by weight (of acid) per million parts by weight of the silicon alkoxide. An acid catalyst concentration of 500 parts per million or less is not adequate to provide the advantage from lowering the PH of the solution in order to form oxide networks and chains which eventually lead to gelation without the formation of colloidal silica.
Liquid silica sols and silica gels are used as starting materials in processes other than the Sol-Gel process for the production of ceramics and glasses. The prior art teaches the preparation of a glass like material with a three-dimensional inorganic network of Si--O, B--O, Sn--O or P--O polyhedra or their mixtures and bound organic residues. The networks are the result of polycondensation of mono-organic silicic acids in aqueous alcoholic solutions. Acid or base catalysts are not used. Control of polymer growth and properties by control of hydrolysis and condensation reactions is not disclosed. The resulting materials combine the properties of glass and plastics, depending on the properties of the organic residues.
There is a need for new oxide glass-ceramics with ultra-low expansivity, low helium permeability and thermal stability over an extended temperature range for applications in optics, electronics and ultra-precision measuring equipment. An example of materials with these characteristics are Lithium aluminum silicate (LAS) glass-ceramics with the stable beta-quartz crystal structure. There are applications where available Lithium Aluminum Silicate glass-ceramics (LAS) such as Corning Glass Works' ULE (type 7971) or Schott's Zerodur are not acceptable. For example, ULE has a high helium permeability and cannot be used for ultra-precision measuring equipment. Because Zerodur has some small instability on thermal cycling near -28.degree. C. and -177.degree. C., which are in the operational temperature range of ultra-precision measurement, it cannot be used for all ultra-precision measurement equipment. Furthermore, the few remaining obtainable LAS glass-ceramic systems are not of a reproducibly acceptable quality and are not easily available. The present invention results from efforts to use the sol-gel process to synthesize materials with specific particle distributions and crystal structures to give specific physical properties such as a low thermal expansion coefficient. sol-Gel synthesis of beta-quartz lithium aluminum silicates and gamma-quartz lithium aluminum silicates have been performed and the material properties have been determined.